Cases of flu increased rapidly over the Christmas period – with higher numbers than during the same period in previous years, according to Public Health England.

The health agency estimates there was a 48% increase in reported cases over a single week with around 1,649 people being struck down.

The rising number of flu cases is thought to be due in part to a rise in two aggressive subtypes attacking the population at the same time.

One is the so-called Aussie flu, also known as the H3N2 subtype, which is a strain of influenza A that triggered two and a half times the normal number of cases in Australia. It mainly affects pregnant women, the elderly and children.

Those with health conditions are also at risk of the bug, which can lead to pneumonia and other health complications.

So far, in Britain, 23 people have died from the flu-outbreak, with nearly a third of fatalities having occurred during Christmas week.

Experts are hoping the flu strain does not prove as deadly to humans as the Hong Kong flu in 1968, which killed one million people.

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Public Health England (PHE) said that, last week, 684 cases of influenza A and 915 of influenza B were recorded, with some 54 cases still yet to be identified. The Aussie flu is also "actively circulating" in Ireland, with at least ten people thought to have lost their lives to the killer virus so far this winter.

According to the PHE data, the number of new cases each week is ten times higher than that of 2015. They are now urging people to get vaccinated against it.

Flu is on the increase (Image: Getty Images)

Can I get a free flu jab?

Most people who get flu will be better within a week and will not be eligible for a free jab. However, certain groups are at a higher risk and more likely to suffer from complications such as pneumonia.

So, a free flu jab is offered by the NHS for those who are:

Aged 65 and over

Aged 18 or over with heart or respiratory problems

Pregnant

Aged six months to two years old if deemed at risk

All children aged two or three, those in reception class and years one to four of primary school can get the flu nasal spray on the NHS. If a child is deemed to be at risk, they can have the spray up to the age of 17.

The only reason that you shouldn't have the jab is if you have had a serious allergic reaction to it in the past. Again, check with your doctor if you are not sure.